Surgically implantable knee prosthesis with captured keel

ABSTRACT

A unicompartmental knee prosthesis for implantation in a knee joint between a femoral condyle and a corresponding tibia plateau is provided including a generally elliptical body having opposed femoral and tibial face, the body having an anterior end and a posterior end. A keel is provided on the tibial face having a generally anterior-posterior orientation, the keel having an anterior end and a posterior end, where the keel posterior end includes a distal posterior portion that extends farther toward the body posterior end compared with a proximal posterior portion of the keel posterior end.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 60/938,012 filed May 15, 2007, which is incorporated by referenceherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a prosthesis which is surgically implantableinto a body joint, such as the knee.

2. Background Art

Current knee arthroplasties typically involve replacement of thearthritic joint surfaces and are known for their use of metal andplastic components. They are usually embedded in apolymethylmethacrylate (PMMA) cement mantle to adhesively andmechanically bond the components to the area of bone exposed during thecourse of surgery.

Typically, this area of exposed bone is 2-3 mm below the area ofexisting eroded bone surface and generally requires removal of theentire subchondral bone in the area of implant location. In all cases,the subchondral (SC) bone of the tibial plateau, which is attached tothe remaining articular surface, is removed as standard practice forboth the total knee (TKR) and partial or unicompartmental knee (UKR)replacement procedures.

Further, FDA guidelines generally dictate that when polyethylene (PE) isused as a bearing surface, whether in conjunction with a metal supportplate or not, at least 6 mm of PE thickness must be used to preventfracturing of the PE during use. When the PE is used on the tibial sideof these implant designs, this requirement leads to bone resections ofthe tibial plateau generally greater than 7 mm. The subchondral bonethickness on a typical tibial plateau is generally 2-3 mm. Thus, atypical TKR or UKR implant will require resection of the entire SC bonepresent on the tibial plateau, leaving only cancellous bone.

The PE is typically held in place by an interference fit or by meltinfusion to a metal backing plate known as the tibial baseplate. Thisbaseplate, in turn, is held in place on the now exposed cancellous boneof the tibia by screws, keels, posts, or combinations of some or all ofthese devices. The screws and keels generally provide immediatefixation, but these are usually enhanced by the addition of the PMMAcement. In the case of perforated keels, tapered and hourglass shapedposts, when these projections are set in uncured cement, the cementforms around and through them and, once hardened, provides an almostindestructible bond between the PMMA cement and the tibial baseplate.The cement also permeates the open cellular structure of the cancellousbone, thus resulting in the same type of bond between the bone and thetibial baseplate. In some unique cases, the metal tibial baseplate isnot used and an all-PE design is bonded directly to the cancellous bonewith the PMMA cement utilizing a roughened PE surface or molded posts tofacilitate the bond with the PMMA cement.

A new generation of tibial hemiarthroplasty (THA) implant designs hasbeen introduced which do not require significant resection of the SCbone of the tibial plateau to function properly. Examples of this arethe U.S. Pat. Nos. 6,206,927; 6,558,421; 6,966,928; 6,866,684; and7,341,602, each of which is incorporated by reference herein. These THAdesigns maintain their proper location in the joint by interference withpreexisting or prepared anatomical shapes present in the knee joint, andnone require PMMA-cemented protrusions or screws for proper function.

Previous keel designs, whether utilized for THA, UKR, or TKR implants,typically utilize an anteriorly-oriented keel. For example, the ZimmerSbarbaro “skate” implant has a keel aligned in the anterior-posterior(AP) direction, with the posterior portion being rounded and sharpenedand the anterior portion having an anterior (forward) pointing distaltip. In order to insert this particular shaped keel into a tibia with acut to accept the keel, the length of the saw cut needed to insert thebottommost portion of the keel would be significantly longer the lengthof the keel at the base of the implant, thus allowing the implant to beable to slide in an anterior fashion upon implantation in an anterior toposterior insertion direction.

Other previous keel designs, such as the DePuy “Preservation” UKR,utilize a keel which extends the majority of the length of thebaseplate. In this implant, the keel also comprises an hourglass shapein medial-lateral (ML) section. Due to the length of the keel and thehourglass design along its length, this implant cannot be inserted intothe tibia without first making a substantial femoral cut to provideaccess to the tibia, or otherwise inserting the implant via a lengthwiseinsertion from the most anterior portion of the tibial plateau. In otherwords, the tibial plateau, rather than receiving a simple angle saw cutin order to receive the implant, must have a milled hourglass shape cutin the plateau which extends through the most anterior cortical bone inorder for the keel to be inserted into the joint. If such a milled cutis not prepared, then a cut equal to the largest width of the keel mustbe made, which would not provide positive locking with the keel unless amantle of cement is used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a prosthesis according to thepresent invention including a keel with angled anterior and posteriorends;

FIG. 2 is a bottom perspective view of the prosthesis of FIG. 1;

FIG. 3 is a side elevational view of a prosthesis according to thepresent invention including a keel with a rounded anterior end andangled posterior end;

FIG. 4 is a bottom perspective view of the prosthesis of FIG. 3;

FIG. 5 is a bottom perspective view of a prosthesis according to thepresent invention including a keel with rounded anterior and posteriorends;

FIG. 6 is a bottom perspective view of a prosthesis according to thepresent invention including a keel with rounded anterior and posteriorends;

FIG. 7 is a side elevational view of a prosthesis according to thepresent invention including a keel with rounded anterior and posteriorends shown with reference to a cross-section of the tibia, wherein arepresentative thickness subchondral and cortical bone are represented;

FIG. 8 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end andchamfered anterior end;

FIG. 9 is a bottom perspective view of the prosthesis of FIG. 8;

FIG. 10 is a side elevational view of the prosthesis of FIG. 8 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 11 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end andoppositely angled anterior end;

FIG. 12 is a bottom perspective view of the prosthesis of FIG. 11;

FIG. 13 is a side elevational view of the prosthesis of FIG. 11 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 14 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end and astep-shaped anterior end;

FIG. 15 is a bottom perspective view of the prosthesis of FIG. 14;

FIG. 16 is a side elevational view of the prosthesis of FIG. 14 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 17 is a side elevational view of a prosthesis according to thepresent invention including a keel with a step-shaped posterior end anda rounded anterior end;

FIG. 18 is a bottom perspective view of the prosthesis of FIG. 17;

FIG. 19 is a side elevational view of the prosthesis of FIG. 17 shownwith reference to a cross-section of a tibia wherein a representativethickness subchondral and cortical bone are represented;

FIG. 20 is a side elevational view of a prosthesis according to thepresent invention including a keel with a step-shaped posterior end anda curved anterior end;

FIG. 21 is a bottom perspective view of the prosthesis of FIG. 20;

FIG. 22 is a side elevational view of the prosthesis of FIG. 20 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 23 is a side elevational view of a prosthesis according to thepresent invention including a keel with a step-shaped posterior end anda chamfered anterior end;

FIG. 24 is a bottom perspective view of the prosthesis of FIG. 23;

FIG. 25 is a side elevational view of the prosthesis of FIG. 23 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 26 is a side elevational view of a prosthesis according to thepresent invention showing generic keel dimensions;

FIG. 27 is a bottom plan view of a prosthesis according to the presentinvention showing generic keel dimensions;

FIG. 28 is a side elevational view of a prosthesis according to thepresent invention showing exemplary keel dimensions;

FIG. 29 is a bottom plan view of a prosthesis according to the presentinvention showing exemplary keel dimensions;

FIG. 30 is a side elevational view of a prosthesis according to thepresent invention including a keel with a hooked posterior end and arelatively longer, angled anterior end;

FIG. 31 is a bottom plan view of the prosthesis of FIG. 30;

FIG. 32 is a front elevational view of the prosthesis of FIG. 30;

FIG. 33 is a rear perspective view of the prosthesis of FIG. 30;

FIG. 34 is a side elevational view of the prosthesis of FIG. 30 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 35 is a top perspective view of a tibial cut which may be utilizedfor receiving the prosthesis of FIG. 30;

FIG. 36 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end and arelatively longer, rounded anterior end;

FIG. 37 is a bottom perspective view of the prosthesis of FIG. 36;

FIG. 38 is a side elevational view of the prosthesis of FIG. 36 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 39 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end and arelatively shorter, rounded anterior end;

FIG. 40 is a bottom perspective view of the prosthesis of FIG. 39;

FIG. 41 is a side elevational view of the prosthesis of FIG. 39 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 42 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end and arelatively shorter, rounded anterior end, the prosthesis including acushioning component on a tibial face thereof;

FIG. 43 is a bottom perspective view of the prosthesis of FIG. 42;

FIG. 44 is a side elevational view of the prosthesis of FIG. 42 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 45 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end and arelatively shorter, rounded anterior end, the prosthesis including acushioning component on a femoral face thereof;

FIG. 46 is a bottom perspective view of the prosthesis of FIG. 45;

FIG. 47 is a side elevational view of the prosthesis of FIG. 45 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 48 is a bottom perspective view of a cushioning component accordingto the present invention;

FIG. 49 is a top perspective view of a femoral face of the prosthesis ofFIG. 45 which is prepared to receive a cushioning component thereon;

FIG. 50 is a side elevational view of a prosthesis according to thepresent invention which may be utilized for a lateral compartmentimplantation, the prosthesis including a keel having an angled posteriorend and a relatively longer, angled anterior end;

FIG. 51 is a bottom perspective view of the prosthesis of FIG. 50;

FIG. 52 is a top perspective view of the prosthesis of FIG. 50;

FIG. 53 is a front elevational view of the prosthesis of FIG. 50;

FIG. 54 is a rear elevational view of the prosthesis of FIG. 50;

FIG. 55 is a side elevational view of the prosthesis of FIG. 50 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 56 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior edge, arounded anterior edge, and an angled cross-keel member;

FIG. 57 is a bottom perspective view of the prosthesis of FIG. 56;

FIG. 58 is a side elevational view of the prosthesis of FIG. 56 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 59 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end, arounded anterior end, and an angled cross-keel member extending thedepth of the keel;

FIG. 60 is a bottom perspective view of the prosthesis of FIG. 59;

FIG. 61 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end, arounded anterior end, and a cross-keel member extending along the depthof the keel;

FIG. 62 is a bottom perspective view of the prosthesis of FIG. 61;

FIG. 63 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end, anangled anterior end, and an angled cross-keel member extending along thedepth of the keel at the anterior end;

FIG. 64 is a bottom perspective view of the prosthesis of FIG. 63;

FIG. 65 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end, ananterior end generally orthogonal to a bottom face of the prosthesis,and a cross-keel member extending along the depth of the keel at theanterior end;

FIG. 66 is a bottom perspective view of the prosthesis of FIG. 65;

FIG. 67 is a side elevational view of a prosthesis according to thepresent invention including a keel with an angled posterior end, arounded, angled anterior end, and a plurality of angled barb membersoriented toward the anterior end;

FIG. 68 is a bottom perspective view of the prosthesis of FIG. 67;

FIG. 69 is a bottom plan view of the prosthesis of FIG. 67;

FIG. 70 is a side elevational view of the prosthesis of FIG. 67 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 71 is a side elevational view of a prosthesis according to thepresent invention including a keel with a hooked posterior end, arounded, angled anterior end, and an angled barb member oriented towardthe anterior end;

FIG. 72 is a bottom perspective view of the prosthesis of FIG. 71;

FIG. 73 is a side elevational view of a prosthesis according to thepresent invention including a keel with a hooked posterior end, arounded, angled anterior end, and an angled barb member oriented towardthe anterior end, wherein the keel tapers at a distal end thereof;

FIG. 74 is a bottom perspective view of the prosthesis of FIG. 73;

FIG. 75 is a rear elevational view of the prosthesis of FIG. 73;

FIG. 76 is a bottom plan view of the prosthesis of FIG. 73;

FIG. 77 is a side elevational view of the prosthesis of FIG. 73 shownwith reference to a cross-section of a tibia, wherein a representativethickness subchondral and cortical bone are represented;

FIG. 78 is a bottom perspective view of a prosthesis according to thepresent invention including a posterior tab, wherein the keel is omittedfor clarity;

FIG. 79 is a top perspective view of the prosthesis of FIG. 78;

FIG. 80 is a side elevational view of the prosthesis of FIG. 78including a keel similar to FIG. 73 and shown with reference to across-section of a tibia, wherein a representative thickness subchondraland cortical bone are represented;

FIG. 81 is a side elevational view of a prosthesis according to thepresent invention including a keel with an expandable portion in themedial-lateral direction, wherein the prosthesis is shown with referenceto a cross-section of a tibia, wherein a representative thicknesssubchondral and cortical bone are represented;

FIG. 82 is a bottom perspective view of the prosthesis of FIG. 81;

FIG. 83 is a bottom perspective view of the prosthesis of FIG. 81 withthe expandable portion actuated;

FIG. 84 is a rear elevational view of the prosthesis of FIG. 83;

FIG. 85 is a side elevational view of a prosthesis according to thepresent invention including a keel and a screw for additional fixation;

FIG. 86 is a bottom perspective view of the prosthesis of FIG. 85;

FIG. 87 is a front elevational view of the prosthesis of FIG. 85;

FIG. 88 is a cross-sectional view of the tibia showing a tibial cuttherein and interaction of the flexed femur with the tibia;

FIG. 89 is a side elevational view of the prosthesis of FIG. 73 as it isinserted into the tibial cut illustrated in FIG. 88;

FIG. 90 is a side elevational view of the prosthesis of FIG. 73 uponfurther insertion into the tibial cut illustrated in FIG. 88;

FIG. 91 is a side elevational view of the prosthesis of FIG. 73 uponcomplete insertion into the tibial cut illustrated in FIG. 88;

FIG. 92 is a schematic representation of a tibia, wherein the lightervolume shown in cross-section represents a typical amount of boneresection required for a prior art unicompartmental knee replacementprocedure;

FIG. 93 is a schematic representation of a tibia, wherein the totalvolume represents a typical amount of bone resection required for aprior art unicompartmental knee replacement procedure, and the upper,lighter volume represents a typical amount of bone resection utilizedfor implanting a prosthesis according to the present invention;

FIG. 94 is a top plan view of an instrument according to the presentinvention which may be utilized for creating a tibial cut in order toimplant a prosthesis according to the present invention;

FIG. 95 is a top perspective view of the instrument of FIG. 94;

FIG. 96 is a side elevational view of the instrument of FIG. 94;

FIG. 97 is a bottom plan view of the instrument of FIG. 94;

FIG. 98 is a top perspective view of a modular instrument according tothe present invention which may be utilized for creating a tibial cut inorder to implant a prosthesis according to the present invention;

FIG. 99 is a top perspective view of the instrument of FIG. 98 where oneportion of the tibial cut guide has been removed;

FIG. 100 is a side elevational view of the instrument of FIG. 98;

FIG. 101 is a top plan view of the instrument of FIG. 98;

FIG. 102 is a bottom plan view of the instrument of FIG. 98;

FIG. 103 is a bottom perspective view of an instrument according to thepresent invention for sizing a prosthesis;

FIG. 104 is a side elevational view of the instrument of FIG. 103;

FIG. 105 is a top perspective view of the instrument of FIG. 103;

FIG. 106 is a side elevational view of an impactor according to thepresent invention in contact with a prosthesis according to the presentinvention shown with reference to a cross-section of a tibia, wherein arepresentative thickness subchondral and cortical bone are represented;

FIG. 107 is a top perspective view of the impactor of FIG. 106;

FIG. 108 is a side elevational view of the impactor of FIG. 106 furtherillustrating a handle thereon;

FIG. 109 is a top perspective view of the impactor and handle of FIG.108; and

FIG. 110 is a wireframe representation of a prosthesis according to thepresent invention including an internal conduit and portal.

DETAILED DESCRIPTION OF THE INVENTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale, andsome features may be exaggerated or minimized to show details ofparticular components. Therefore, specific structural and functionaldetails disclosed herein are not to be interpreted as limiting, butmerely as a representative basis for teaching one skilled in the art tovariously employ the present invention.

The present invention includes a prosthesis with a captured keel designwhich provides for positive interlocking that resists unintendeddislodgement of the implant without the need for PMMA cement. Theprosthesis according to the present invention may be used in conjunctionwith a tibial preparation that removes much of the remaining cartilagebut leaves the majority of the SC bone of the tibial plateau intact. Inaccordance with the present invention, and differentiated from priorkeel designs, the keel disclosed herein incorporates a notched, angled,or other design wherein the most distal, posterior portion of the keelmay extend more posteriorly than the most proximal, posterior portion ofthe keel, thus providing a keel shape of negative draft. In addition,the distal end of the keel may be longer in an AP direction than theproximal end of the keel, giving a recessed or hooked appearance. Thekeel may be of sufficient depth such that while the bottom of theprosthesis sits predominantly on the existing SC bone, the extendedportion of the keel may reach below the underside of the SC bone, thuscapturing the prosthesis with the remaining SC bone.

By leaving the majority of the SC bone intact and undisturbed inaccordance with the present invention, the risk over time of prosthesissubsidence into the tibia, the primary failure mode of present day UKRimplants, may be largely eliminated. This may be of great significanceto patients who have unicompartmental disease but are contra-indicatedfor UKR or TKR procedures because of gross obesity. Further, unlikeother THA implants, the captured keel prosthesis according to thepresent invention may provide for significantly reduced motion againstthe tibia because of the interlocking keel design.

One function of the prosthesis according to the present invention may beto effectively replace the articular material that has been lost due tothe effects of osteoarthritis by spanning the diseased area andsupporting the prosthesis by intimate contact with the surroundinghealthy tissue. An advantage of this approach is that the combination ofremoving healthy articular material and replacing that same materialwith the prosthesis allows for a minimal thickness prosthesis to beutilized which does not need to disturb the meniscal function orlocation. This approach may result in an area under the prosthesis wherethe prosthesis is barely in contact with the area of osteoarthritis(where SC bone has been deformed or eburnated). The prosthesis accordingto the present invention may not disturb SC bone, thus reducing thechance of any prosthesis subsidence into cancellous bone like a UKRbaseplate often does.

In accordance with the present invention, unlike previous THA and UKRmethods, it is not necessary that the tibial plateau have an absolutelyflat surface after surgical preparation. Rather, the majority of theplateau, once the remaining articular material has been largely removed,may provide an adequate peripheral shoulder on which the prosthesis canbe supported. Thus, if the area of the osteoarthritis defect were stilllower than the SC bone once the majority of the plateau has beenflattened, the prosthesis may simply bridge this area while the keel maypass through the defect to the underside of the SC bone in that area,providing an interlocking behavior.

The keel of the prosthesis according to the present invention isarranged to be at least partially received within a cut prepared on thetibial plateau. According to one aspect of the present invention, thetibial cut may be of a size substantially equal to the size of aproximal end of the keel right underneath the prosthesis, such thatlittle or no give exists between the tibial cut and keel, such that useof cement may be avoided. The desired location and size of the tibialcut may be matched with a particular prosthesis selected from a libraryof prostheses having different locations and sizes of keels. The tibialcut may be prepared with an appropriate milling device or the like whichmay be accurately located via temporary fixation, computer guidance, orother means.

The surgical procedure may involve resection (flattening) of theremaining articular material on the tibial plateau in the area where theprosthesis will reside using an oscillating saw or other tool, sizingthe plateau for the proper length, width and thickness and, utilizing acutting guide, making a saw or rasp cut at the proper angle anddirection with the oscillating saw. This vertical saw cut may ultimatelydetermine the final position of the prosthesis. The keel of theprosthesis may be positioned into the guided saw cut location with theknee flexed and once in position, gently hammered into place. Inaddition to not requiring the removal of SC bone, the prosthesisaccording to the present invention does not require femoral resection toimplant.

While the prosthesis according to the present invention is shown anddescribed herein as being implanted in a knee joint, specifically as aunicompartmental knee prosthesis implantable in a knee joint between afemoral condyle and a corresponding tibial plateau, it is understoodthat the prosthesis could be utilized in joints other than the knee suchas, but not limited to, the hip, shoulder, wrist, ankle, or elbow, orother small joints of the foot or hand.

With reference to the figures, the prosthesis according to the presentinvention, designated generally by reference numeral 10, comprises abody 12 which may be generally elliptical and which includes a bottom,or tibial, face 14 and an opposed top, or femoral, face 16. Body 12includes an anterior end 18 and a posterior end 20, corresponding to theanatomical location of these ends 18, 20 of body 12 upon implantationinto the knee joint, wherein the prosthesis shape may generally coverthe majority of the medial or lateral tibial plateau T. To restrainmovement of the prosthesis, a keel 22 may be provided on the bottom face14, and may have a generally AP orientation as depicted herein.According to the present invention, the keel 22 can have any location onthe bottom face 14 and can be of any size suitable for insertion. Keel22 has an anterior end 24 and a posterior end 26, again according to theanatomical location of these ends 24, 26 upon implantation. Keelposterior end 26 may include a distal posterior portion 28 that extendsfarther toward the body posterior end 20 compared with a proximalposterior portion 30 of the keel posterior end 26, creating a posteriorkeel design which is relieved, undercut, hooked, or similar. The keeldesigns according to the present invention provide inherent stability tothe prosthesis 10 because femoral loading on the prosthesis 10 cannotreproduce motion of the prosthesis 10 required to dislodge it from thetibial plateau T. In addition to the embodiments depicted herein, it isunderstood that any keel having a distal posterior portion extendingfurther toward the body posterior end than does the proximal posteriorportion of the keel is fully contemplated according to the presentinvention.

The top face 16 could be of uniform shape or could have a combination ofsloped and flat surfaces. The entire top face 16 or portions thereof mayrange from generally convex to generally concave or combinations ofthose surfaces, and range from generally conformal to non-conformal,depending on the compartment for implantation, the condition of theligaments and other soft tissue structure at the time of surgery, andhow much stability the knee will require. The femoral face 16 shape maybe characterized as an aspect ratio defined by the chord line and thethickness above or below this chord line as a function of distance froma defined point on the chord line, such as the leading edge or midpoint,much like an airfoil can be described. It is understood that the terms“concave” and “convex” as used herein are not restricted to describingsurfaces with a constant radius of curvature, but rather are used todenote the general appearance of the surface.

According to one aspect of the present invention, the remainder of thebottom face 14, excluding the keel 22, may include, for example, agenerally flat surface which does not require “seating.” However it isunderstood that other contours of the bottom face 14 are fullycontemplated in accordance with the present invention. For example,depending upon the compartment of implantation, the condition of theligaments and other soft tissue structure at the time of surgery, andhow much stability the knee will require, the bottom face 14 may begenerally concave, flat, or convex, or anywhere within the range fromconcave to convex or combinations of those surfaces. Again, it isunderstood that the terms “concave” and “convex” as used herein is notrestricted to describing a surface with a constant radius of curvature,but rather is used to denote the general appearance of the surface.

The body 12 further includes a peripheral edge 32 extending between thebottom face 14 and the top face 16. Edges along the periphery of theprosthesis 10 can be rounded. Any thickness of the prosthesis 10 orvariation of thickness within the prosthesis 10 may be utilized, and maybe determined so as to provide proper joint tensioning throughout therange of motion of the knee. The prosthesis 10 according to the presentinvention may have length and width proportions roughly similar to anyof the current UKR tibial base plates, whereas its thickness maygenerally be 2-3 mm less than the UKR overall baseplate/PE thicknesssince the SC bone is not being removed. Of course, prosthesis 10 is notlimited to these dimensions. The prosthesis 10 according to the presentinvention may be used in conjunction with the remaining meniscus ormeniscal replacement by having a relieved thickness along the peripherywhere the meniscus is located. Additionally, the posterior end 20 of thefemoral face 16 may be tapered, and two different femoral and tibialsurface profiles utilized. Thinning of the posterior end 20 may behelpful in deep flexion to eliminate a lever which could tip theprosthesis 10 upward and potentially out of engagement with the tibialplateau T, and also to relieve possible impingement and pain.

It is understood that the term “generally elliptical” is intended toinclude all construction methods which yield a generally planar shapewhich is longer in one direction than in the transverse direction andhas generally rounded corners, and that the prosthesis 10 is nototherwise limited to any particular shape.

With reference first to FIGS. 1-2, a prosthesis 10 according to thepresent invention is depicted including a keel 22 with anterior andposterior ends 24, 26 which are angled toward the body posterior end 20such that the distal posterior portion 28 of the keel 22 extends farthertoward the body posterior end 20 compared with the proximal posteriorportion 30 of the keel 22. This creates an undercut portion at the keelposterior end 26 which may then engage underneath the SC bone uponinsertion of the prosthesis 10. Insertion of the prosthesis 10 may befacilitated by rounding of a distal anterior portion 34 of the keel asshown in FIGS. 3-4, or rounding of both the distal anterior 34 anddistal posterior portions 28 of the keel 22 as depicted in FIGS. 5-7.Alternatively, solely the distal posterior portion 28 of the keel may berounded.

As shown in FIGS. 5-6, keel 22 may be embodied as having differentlengths along tibial face 14 and be positioned differently on tibialface 14. For example, FIG. 5 depicts a keel 22 having a length thatextends along approximately half the length of the tibial face 14,positioned toward body anterior end 18, whereas FIG. 6 depicts a keel 22having a length that extends along approximately 20% of the length ofthe tibial face 14. Of course, it is understood that any length, depth,and positioning of keel 22 with respect to tibial face 14 is fullycontemplated.

In accordance with another embodiment of the present invention, FIGS.8-10 depict a prosthesis including a keel with an angled posterior end26 and chamfered distal anterior portion 34. FIGS. 17-25 illustrate keelembodiments according to the present invention wherein the keelposterior end 26 includes a notched or step-shaped configuration, suchas to form an approximately 90° angle at the keel posterior end 26, andthe keel anterior end 24 is angled, rounded, chamfered, or a combinationthereof.

With reference to FIGS. 11-16, in addition to the angled posterior end26 described above, the keel 22 according to the present invention couldalso incorporate an oppositely angled, notched, or step-shaped anteriorend 24 wherein the keel anterior end 24 includes a proximal anteriorportion 36 that extends farther toward the body anterior end 18 comparedwith a distal anterior portion 34 of the keel anterior end 24. Thus,once engaged with the underside of the SC bone, the prosthesis 10 may besecured both anteriorly and posteriorly to preventing tipping oraccidental dislodgement of the prosthesis 10.

Turning now to FIGS. 26-27, a prosthesis 10 according to the presentinvention is illustrated with generic keel dimensions for a left medialknee prosthesis. The keel position may be described as a percentage ofthe length ahead of or behind the prosthesis centerline. The depth maybe measured at the longest point as measured from the bottom face 14 ofthe prosthesis 10. A relationship table for the dimensions shown isprovided below, where it is understood a change in these ratios by+/−25% or more is fully contemplated according to the present invention.

D188=Length

D29=0.367*D188

D76=0.224*D188

D95=0.510*D188

D97=0.694*D188

D103=0.061*D188

D104=0.061*D188

D176=0.204*D188

D177=0.510*D188

D178=0.735*D188

D181=0.041*D188

D182=0.061*D188

D189=0.551*D188

D183=0.429*D188

D186=0.429*D188

D175=0.200*D188

These ratios may describe the relative placement of the keel 22 on theprosthesis 10 (along the anterior-posterior and medial-lateraldirections) and may apply to all keel embodiments shown and describedherein, wherein the variations in the design of the keel itself aredepicted in the drawings. FIGS. 28-29 illustrate possible dimensions foran exemplary prosthesis according to the present invention having a 49mm length and 2 mm thickness. It is understood, of course, that theprosthesis 10 is not limited to this configuration.

U.S. Pat. No. 6,966,928, incorporated by reference herein, describes akeel having a depth which tapers from one end of the prosthesis toanother, such that the taper may be used to facilitate the insertion ofthe prosthesis. Such a tapered design may also be utilized with theprosthesis 10 according to the present invention. In particular, FIGS.30-34 illustrate a prosthesis 10 including a keel 22 with a hookedposterior end 26 and a relatively longer, angled anterior end 24 suchthat the keel anterior end 24 extends longer distally compared with thekeel posterior end 26. FIG. 35 is a top perspective view of a tibial cut68 which may be utilized for receiving the prosthesis 10 of FIG. 30.FIGS. 36-38 depict a prosthesis 10 according to the present inventionincluding a keel 22 with an angled posterior end 26 and a relativelylonger, rounded anterior end 24. FIGS. 39-41 depict a prosthesis 10according to the present invention including a keel 22 with an angledposterior end 26 and a relatively shorter, rounded anterior end 24, suchthat the keel posterior end 26 extends longer distally compared with thekeel anterior end 24. In addition, the keel 22 in any embodimentdepicted herein may taper in width from an end proximal to theprosthesis bottom face 14 to an end distal from the prosthesis bottomface 14 such that a proximal end 38 of the keel 22 is wider than adistal end 40 of the keel 22, creating a sort of knife edge which mayfacilitate insertion.

In accordance with the present invention, a thinner prosthesis may beused where the final intent is to cover at least one face of the baseprosthesis with a load-absorbing, cushioning, or other surfacingcomponent 42. As shown in FIGS. 42-44, one embodiment may utilize a hardarticulating surface bearing material like metal, ceramic, or certainpolymers (e.g., pyrolytic carbon or PEEK) which may include a surfacingcomponent 42 provided on a bottom face 14 thereof wherein the surfacingcomponent 42 then contacts the tibial plateau T, or alternatively issandwiched between the outer articulating surface and an anchoringimplant base. The load absorbing material may be a polymer or othermaterial, such as a metallic sponge or springs. Biologically compatibleurethanes, various hydrogels, and/or polymers that contain biologiccomponents can also be utilized. It is also possible that one materialcan perform both the articulating function and the load absorbingfunction. According to one aspect of the present invention, theprosthesis configuration may allow for volume expansion of the surfacingcomponent 42 while under load. The prosthesis 10 and the surfacingcomponent 42 may be mechanically linked at the time of surgery to allowfor surgeon selection of polymer thickness and material properties suchas water content, durometer, viscoelastic behavior, and others. However,such a linkage is not necessary.

With reference to FIGS. 45-47, a prosthesis 10 according to the presentinvention is illustrated which includes a load absorbing, cushioning, orsurfacing component 42 on a top face 16 thereof. In both this embodimentand that described above, a surface of the prosthesis 10 may be preparedmechanically and/or chemically to receive the surfacing component 42.For example, FIG. 48 illustrates a bottom perspective view of asurfacing component 42 according to the present invention, and FIG. 49illustrates a top perspective view of a top face 16 of the prosthesis ofFIG. 45 which is prepared to receive a surfacing component 42 thereon.In addition, the keel 22 itself could have a load absorbing, cushioning,or surfacing component associated therewith, such as to provide strainisolation.

With reference now to FIGS. 50-55, a prosthesis 10 according to thepresent invention is illustrated which may be utilized for a lateralcompartment implantation. The prosthesis depicted includes a keel 22having an angled posterior end 26 and a relatively longer, angledanterior end 24, although any of the keel 22 embodiments shown ordescribed herein could alternatively be utilized. As shown, a posteriorslope 44 may be provided on both the femoral and tibial faces 14, 16 ofthe prosthesis 10.

The prosthesis 10 according to the present invention may also include across-keel 46 provided generally in the medial-lateral (ML) direction,wherein cross-keel 46 may have a shorter length in the ML direction thandoes keel 22 in the AP direction. Such a cross-keel 46 may enhance thestability of the prosthesis 10 once inserted. The cross-keel 46 may begenerally rectangular in shape, but is not limited as such. FIGS. 56-58illustrate a prosthesis 10 according to the present invention includinga keel 22 with an angled posterior end 26, a rounded anterior end 24,and a cross-keel 46 positioned at approximately the midpoint of the keel22 in the AP direction, wherein the cross-keel 46 extends approximately½ the depth of the keel 22. In this case, cross-keel member 46 has anangle which is similar to the angle of the keel posterior end 26,wherein a distal portion 48 of the cross-keel 46 extends farther towardthe body posterior end 20 compared with a proximal portion 50 of thecross-keel 46. Of course, other depths and orientations of cross-keel 46as compared with keel 22 are also contemplated. FIGS. 59-60 depict aprosthesis 10 according to the present invention including a keel 22with an angled posterior end 26, a rounded anterior end 24, and anangled cross-keel 46 extending to approximately the same distal depth asthe keel 22. FIGS. 61-62 illustrate a prosthesis 10 according to thepresent invention including a keel 22 with an angled posterior end 26, arounded anterior end 24, and a cross-keel member 46 extending along thedepth of the keel 22 generally orthogonal to the prosthesis bottom face14. FIGS. 63-64 depict a prosthesis 10 according to the presentinvention including a keel 22 with an angled posterior end 26, an angledanterior end 24, and an angled cross-keel member 46 extending along thedepth of the keel 22 at the anterior end 24. FIGS. 65-66 illustrate aprosthesis 10 according to the present invention including a keel 22with an angled posterior end 26, an anterior end 24 extending distallygenerally orthogonal to bottom face 14, and a cross-keel member 46extending along the depth of the keel 22 at the anterior end 24generally orthogonal to the prosthesis bottom face 14.

The prosthesis 10 according to the present invention may also includeshorter cross-keels or barb members 52 protruding from keel 22 generallyin the ML direction. For example, FIGS. 67-70 illustrate a prosthesis 10according to the present invention including a keel 22 with an angledposterior end 26, a rounded, angled anterior end 24, and a plurality ofangled barb members 52 having an orientation generally orthogonal to theangle of the posterior end 26, where each barb member 52 has a distalportion 54 that extends farther toward the body anterior end 18 comparedwith a proximal portion 56 of the barb member 52. Barb members 52 may betapered such that an end 58 adjacent the keel 22 is wider compared withan end 60 removed from the keel 22. In another embodiment, a singleangled barb member 52 may be utilized as in FIGS. 71-77. In thisexample, the prosthesis 10 may include a keel 22 with a hooked posteriorend 26 and a rounded, angled anterior end 24, although it is understoodthat barb members 52 may be used with any keel design shown or describedherein. As illustrated in FIGS. 73-77, the keel 22 may taper so as to bemore narrow at an end 40 thereof distal from the prosthesis bottom face14 such that the bottommost portion of the keel 22 may be sharpened,which may be helpful in the downward and backward motion used to insertthe prosthesis 10. The hooked posterior end 26 may also taper to athree-sided point. This configuration may help facilitate securing theprosthesis 10 and capturing SC bone. According to one aspect of thepresent invention, the more proximal portion 30 of the posterior end 26may remain flat in order to avoid an upward cutting capability of theprosthesis 10 once inserted.

In further accordance with the present invention, FIGS. 78-80 illustratea prosthesis 10 including a posterior tab 62 provided at body posteriorend 20 and extending distally beyond tibial face 14 which may be used toprovide further stability to the prosthesis 10 once seated on the tibialplateau T. It is understood that the posterior tab 62 may have any shapeor depth suitable for implantation, and is not limited to theconfiguration depicted herein.

The gap between the femoral condyle and the tibial plateau, after theplateau has been prepared, determines the allowable size and particularshape of the prosthesis that can be fit into this space. The location ofthe keel on the prosthesis, the angle of the keel, and the overalllength and/or depth of the keel may determine the allowable insertionangle and thus the overall thickness of the prosthesis, where too largea keel or too posterior a keel location may prevent insertion of theprosthesis. To solve this problem, a deformable keel 22 could beutilized. Alternatively, a short depth (e.g., 2-3 mm) keel 22 could beused. In this case, once the prosthesis 10 is located in position,screws may be placed down through the interior of the keel 22 for finalfixation.

As described above, a keel that is integral with the prosthesis body maybe limited in length due to insertion issues. A post or screw may beadded to the prosthesis, such as by threading it through the prosthesisbody, once the prosthesis is in position. “Captured” screws have a lowertapered threaded portion for grabbing the bone and an upper portion witha machined thread for attaching to the prosthesis body during the lastportion of travel of the screw into the bone, allowing for additionalstability for the prosthesis. Strain isolation bushings may be addedbetween the prosthesis and the screw to further isolate the screw fromany strain induced by micromotion of the prosthesis. According to oneaspect of the present invention, a modular keel assembly could beimplemented, where a greater depth keel may be inserted once a shortdepth keel prosthesis is in place. This approach has the added advantageof customized fits for an individual patient's needs. In yet anotherembodiment, the prosthesis 10 may include an expandable keel portion 64such as, but not limited to, an anterior portion as depicted in FIGS.81-82. Once the prosthesis 10 is in place, a push pin or other actuatorcould be advanced through an internal slot in the prosthesis 10 to flareout the expandable keel portion 64 in the ML direction for additionalcapture as depicted in FIGS. 83-84.

According to the present invention, at least a portion of the keel 22,especially those portions that will ultimately reside in cancellousbone, can be coated to promote bony in-growth or left smooth todiscourage it. The keel 22 may include one or more openings therein. Theprosthesis 10 according to the present invention could be screwed in, orhave any type of fixation (e.g., cement) for additional stability. Forexample, with reference to FIGS. 85-87, a screw 66 or other fastener maybe provided at the anterior-lateral corner of the prosthesis 10, and maybe angled roughly 30 degrees downward off of the plane of the tibialplateau and 30 degrees from the AP direction of the keel 22, laterally(towards the tibial eminence). Of course, a screw or screws 66 are notlimited to this position or orientation with respect to the prosthesis10.

FIG. 88 is a cross-sectional view of the tibia T showing a tibial cut 68therein and interaction of the femur with the tibia T, and FIGS. 89-91illustrate the prosthesis of FIG. 73 as it is inserted into the tibialcut 68. As shown, the prosthesis 10 may be inserted at approximately a45 degree angle and, in theory, could only be potentially dislodged viathe same path. However, were the prosthesis 10 to come upward andforward, the femur in extension would push the prosthesis back intoplace, thus providing inherent stability.

The prosthesis 10 according to the present invention can be a monolithicdesign or may be made of two or more separate components. By utilizing amodular design, the physician may be able to draw from a library ofcomponents at the time of surgery that may adhesively, mechanically,magnetically, or otherwise cooperate with each other to yield anassembled prosthesis particularly suited for that particular patient'sknee geometry, and also maintain a desired balance between the extensionand flexion gap throughout the range of motion. An additional benefit ofsuch a prosthesis may be that the components could be assembled in thejoint space. This modularity of the prosthesis of the present inventionmay also allow the physician to implant a more standard first componentwhile providing flexibility in the selection of a corresponding secondcomponent that may be best suited for each individual patient.

The prosthesis 10 according to the present invention may comprise arelatively hard, relatively high modulus, low friction material.Suitable materials include, for example, metals such as steel ortitanium, metal alloys, ceramics, and reinforced and non-reinforcedthermoset or thermoplastic polymers. The material of construction may bechosen such that the top face spans defects in the femur withoutdeforming into the defects, allowing for the provision of recessed ornon-contacting areas of the prosthesis to encourage articularregeneration. In the case of a modular prosthesis, the components neednot be formed of the same material. For example, a first component maybe relatively hard, whereas a conformal second component may beconstructed from a relatively lower modulus material to allow for somedeformation. Furthermore, the prosthesis 10 need not be made only of asingle material. Rather, the prosthesis 10 or components thereof mayeach have areas of lower or higher modulus material, and compositestructures of steel/thermoplastic, steel/ceramic, ceramic/polymer, orothers may be used.

In greater detail, materials of construction could include, but are notlimited to, elastomeric polymers such as nylon, silicone, polyurethane,polypropylene, polyester, or the like, optionally fiber-reinforced, orviscous-elastic materials such as PVA hydrogels, as well as otherhydrophilic materials or hydrophobic materials. Polymers capable ofcontaining living cells could also be utilized. Still other possiblematerials are those which can replicate the function of naturallyoccurring cartilage or meniscus. A surface coating can be employed, suchas for the reduction of friction between the prosthesis and the femoralcondyle. Generally, the areas of the prosthesis 10 expected to have themost wear may be made of stronger, more abrasion resistant material thanthe remainder of the prosthesis when composite structures are used. Assuch, it is understood that particular areas may be softer than thematerial used for constructing the majority of the prosthesis 10.

In accordance with the present invention, the prosthesis 10 may bemanufactured so as to substantially contain, or have deposited thereon,a biologically or pharmaceutically active material such as, for example,one that promotes tissue regrowth, retards tissue degeneration, ordecreases inflammation. This may be particularly suitable when theprosthesis functions to bridge a defective area of bone or articularcartilage. The active material may be provided in the form of a coatinganywhere on the prosthesis 10, or may be contained within the prosthesisin the form of a solid, liquid, gel, paste, or soft polymer material.Such active materials may be designed to be delivered at once or in atimed-release manner.

It is known that the erosion of articular cartilage that occurs in anosteoarthritic patient exposes the subchondral bone, often known aseburnation. Nociceptor endings of small diameter axons (nerve endings)that are present in the bone via the bone marrow are now subject toactivation by biomechanical forces associated with weight bearing.Further, higher than normal intraosseous pressure (fluid pressurepresent in the cancellous (trabecular) and highly mineralized(subchondral) bone) are known to exist in a large percentage of OApatients. The combination of these events is considered a likely sourceof a large amount of the pain felt by a patient with OA.

Biomechanical forces causing pain would likely occur during activity. Ina procedure known as percutaneous vertebroplasty, bone cement, usuallyPMMA, is injected into the cavitated vertebral body that has partiallyor is at risk of collapsing. The injected cement hardens and increasesthe mechanical strength of the bone. As expected, bone deformation underload is decreased and reduces the mechanical forces applied to thenociceptive nerve endings and further, the PMMA is known to be toxic fornerve tissue and this procedure causes at least a partial denervation ofthe bone matrix, yielding immediate pain relief for the patient.

Higher than normal fluid pressure in the bone is suspected as the causefor the “bone-throbbing” pain often felt at night by these patients.Transplant patients taking cyclosporine, a known vasoconstrictor, aresubject to severe, episodic knee pain in the absence of any apparentarticular pathology. This phenomenon is readily controlled, however, byadministration of a vasodilator, nifedipine.

According to the present invention, when the subchondral bone isperforated in preparation for the keel 22, immediate reduction ofintraosseous pressure may be noted because of the obvious bleeding thatoccurs. If the keel 22 and perhaps the bottom face 14 are coated with avasodilator such as nifedipine or similar acting pharmaceutical agents,one can expect continued reduction of the intraosseous pressure. Timedelay of this medication may be utilized so that short term healing ofthe bone lesion can occur without continued bleeding and, once healed,maintain the vasodilation activity for an extended period of time.

Further, if additional nerve-targeting agents that are known to be toxicto nerve fibers (i.e., PMMA) or have the ability to desensitize thenerve endings through overstimulation (i.e., capsaicin) can also beadded to the prosthesis 10, especially in that area of the prosthesis 10with the most direct access to these fibers, such as the keel 22. Theseagents would be intended to disperse or leach out of the prosthesis 10itself via a coating added to the prosthesis 10 or held in a pocketedreservoir within the prosthesis 10. Combinations of such agents would bethe likely methodology, whether mixed together, applied to separateregions of the prosthesis 10, or having dual functionality.

Finally, with reference to FIG. 110, the prosthesis 10, at some pointafter the initial implantation, having expended its reservoir of suchagents, can also contain at least one internal conduit 90 that wouldallow for a surgeon to reapply the agents via a portal 92 that isaccessible from the exposed edge of the prosthesis 10 and leads to thatregion of the keel 22 below the topmost surface of the subchondral boneand perhaps to the underside of the prosthesis 10 that rests on thesubchondral bone. Applications of such active agents to any keeled orposted implant as used in the hip, thumb, big toe, vertebra and otherjoints in the body would have similar function and pain relievingpurpose.

Turning now to FIG. 92, a schematic representation of a tibia T isdepicted wherein the lighter volume shown in cross-section represents atypical amount of bone resection required for a prior artunicompartmental knee replacement procedure. FIG. 93 is a schematicrepresentation of a tibia T, wherein the total volume represents atypical amount of bone resection required for a prior artunicompartmental knee replacement procedure, and the upper volume shownin white represents the lesser amount of bone resection utilized forimplanting a prosthesis 10 according to the present invention.

FIGS. 94-97 depict an instrument 70 according to the present inventionwhich may be utilized for creating a tibial cut in order to implant aprosthesis 10 according to the present invention. The instrument 70shown may be placed on top of the flattened tibial plateau and may bepinned or otherwise secured in place, such as using the illustrated hole72. The instrument 70 also includes a tibial cut guide 74 having a slot76 which may be formed at an angle that corresponds to the angle of thekeel of the prosthesis to be implanted. FIGS. 98-102 illustrate amodular instrument 70 according to the present invention which may beutilized for creating a tibial cut in order to implant a prosthesisaccording to the present invention. In particular, FIG. 99 is a topperspective view of the instrument of FIG. 98 where one portion of thetibial cut guide 74 has been removed. The modular instrument 70 may thenbe assembled with different guide components for generating cuts fordifferent sizes and locations of a keel to be inserted. FIGS. 103-105depict an instrument 78 according to the present invention for sizing atibial cut, so as to ensure that the cut has been made correctly for theprosthesis to be implanted. Lastly, FIGS. 106-109 depict an impactor 80according to the present invention in contact with a prosthesis 10according to the present invention shown with reference to across-section of a tibia T. In accordance with one aspect of the presentinvention, the impactor 80 may be configured to engage the prosthesis 10toward the middle thereof, and the handle 82 may have the ability topivot as the prosthesis 10 is being inserted.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A unicompartmental knee prosthesis for implantation in a knee jointbetween a femoral condyle and a corresponding tibia plateau, theprosthesis comprising: a generally elliptical body having opposedfemoral and tibial faces, the body having an anterior end and aposterior end; and a keel provided on the tibial face having a generallyanterior-posterior orientation, the keel having an anterior end and aposterior end, wherein the keel posterior end includes a distalposterior portion that extends farther toward the body posterior endcompared with a proximal posterior portion of the keel posterior end. 2.The prosthesis according to claim 1, wherein the distal posteriorportion of the keel posterior end is rounded.
 3. The prosthesisaccording to claim 1, wherein the keel posterior end is step-shaped toform an approximately 90° angle at the keel posterior end.
 4. Theprosthesis according to claim 1, wherein a distal end of the keel islonger than a proximal end of the keel.
 5. The prosthesis according toclaim 1, wherein the keel anterior end includes a proximal anteriorportion that extends farther toward the body anterior end compared witha distal anterior portion of the keel anterior end.
 6. The prosthesisaccording to claim 5, wherein the distal anterior portion of the keelanterior end is rounded.
 7. The prosthesis according to claim 5, whereinthe distal anterior portion of the keel anterior end is chamfered. 8.The prosthesis according to claim 1, wherein the keel anterior endincludes a distal anterior portion that extends farther toward the bodyanterior end compared with a proximal anterior portion of the keelanterior end.
 9. The prosthesis according to claim 8, wherein the keelanterior end is step-shaped to form an approximately 90° angle at thekeel anterior end.
 10. The prosthesis according to claim 1, wherein thekeel posterior end is hook-shaped to form an obtuse angle at the keelposterior end.
 11. The prosthesis according to claim 1, wherein the keelanterior end extends longer distally compared with the keel posteriorend.
 12. The prosthesis according to claim 1, wherein the keel posteriorend extends longer distally compared with the keel anterior end.
 13. Theprosthesis according to claim 1, wherein the keel tapers such that aproximal end of the keel is wider than a distal end of the keel.
 14. Theprosthesis according to claim 1, further comprising a surfacingcomponent provided on at least one of the tibial and femoral faces. 15.The prosthesis according to claim 1, further comprising a cross-keelhaving a generally medial-lateral orientation.
 16. The prosthesisaccording to claim 15, wherein the keel extends longer distally comparedwith the cross-keel.
 17. The prosthesis according to claim 15, whereinthe cross-keel extends to approximately the same distal depth as thekeel.
 18. The prosthesis according to claim 15, wherein a distal portionof the cross-keel extends farther toward the body posterior end comparedwith a proximal portion of the cross-keel.
 19. The prosthesis accordingto claim 15, wherein the cross-keel extends distally generallyorthogonally from the tibial face.
 20. The prosthesis according to claim15, wherein the cross-keel extends distally along the keel anterior end.21. The prosthesis according to claim 1, the keel further comprising atleast one barb member protruding therefrom and having a generallymedial-lateral orientation, the barb member having a distal portion thatextends farther toward the body anterior end compared with a proximalportion of the barb member.
 22. The prosthesis according to claim 21,wherein the at least one barb member tapers such that an end adjacentthe keel is wider compared with an end removed from the keel.
 23. Theprosthesis according to claim 1, further comprising a tab extendingdistally from the posterior end of the body.
 24. The prosthesisaccording to claim 1, wherein the keel includes a portion which isexpandable in a generally medial-lateral direction.
 25. The prosthesisaccording to claim 24, wherein the expandable portion is providedadjacent the keel anterior end.
 26. The prosthesis according to claim 1,wherein the body is arranged to receive a fastener.
 27. The prosthesisaccording to claim 1, wherein the keel is constructed from a deformablematerial.
 28. The prosthesis according to claim 1, wherein theprosthesis includes a biologically or pharmaceutically active materialassociated therewith.
 29. The prosthesis according to claim 28, whereinthe biologically or pharmaceutically active material is a vasodilatorassociated with the keel.
 30. The prosthesis according to claim 28,wherein the biologically or pharmaceutically active material is anerve-targeting agent associated with the keel.
 31. The prosthesisaccording to claim 1, further comprising an internal conduit from thebody to the keel.
 32. A unicompartmental knee prosthesis forimplantation in a knee joint between a femoral condyle and acorresponding tibia plateau, the prosthesis comprising: a generallyelliptical body having opposed femoral and tibial faces, the body havingan anterior end and a posterior end; and a keel provided on the tibialface having a generally anterior-posterior orientation, the keel havingan anterior end and a posterior end, wherein the keel posterior endincludes a distal posterior portion that extends farther toward the bodyposterior end compared with a proximal posterior portion of the keelposterior end, and wherein the keel anterior end includes a proximalanterior portion that extends farther toward the body anterior endcompared with a distal anterior portion of the keel anterior end.
 33. Amethod of implanting a unicompartmental knee prosthesis in a knee jointbetween a femoral condyle and a corresponding tibia plateau, the methodcomprising: providing a generally elliptical body having opposed femoraland tibial faces, the body having an anterior end and a posterior end,the prosthesis including a keel provided on the tibial face having agenerally anterior-posterior orientation, the keel having an anteriorend and a posterior end, wherein the keel posterior end includes adistal posterior portion that extends farther toward the body posteriorend compared with a proximal posterior portion of the keel posteriorend; and creating a cut in the tibial plateau arranged to at leastpartially receive the keel.
 34. The method according to claim 33,wherein a majority of subchondral bone of the tibial plateau is leftintact.
 35. The method according to claim 33, further comprisingpositioning the keel to reach below an underside of subchondral bone ofthe tibial plateau, thus capturing the prosthesis.
 36. The methodaccording to claim 33, further comprising associating a biologically orpharmaceutically active material with the prosthesis.
 37. The methodaccording to claim 36, further comprising filling the prosthesis withthe active material via an internal conduit between the body and thekeel.